This invention relates to a stretch bender for bending a strip work piece by thrusting it against a bending die while applying axial tension to the work piece.
When a window frame for an automobile is formed, a steel strip is rolled, cut and bent into a desired shape. In order to form a work piece into a three-dimensional shape which is fitted to the outer shape of the window, a stretch bender for three-dimensional bending is required.
Schematic constitution of such a stretch bender is illustrated in FIGS. 11A and 11B. The stretch bender 400 comprises a base 410, curved rails 420 mounted on the base 410, arms 440 which rotatively move on the curved rails 420 around support shafts 430 provided on the base 410, pillars 450 movably standing on the arms 440, lifts 460 which can be moved upward and downward guided by the pillars 460, and chuck mechanisms 470 provided on the lifts 460 for gripping a strip work piece. In stretch bending, the chuck mechanisms 470 grip both ends of the strip work piece 480, and while tension is applied to the work piece 480 to its axial direction, the work piece 480 is thrust against the bending die 490. The bending die 490 has a groove for bending, and the work piece 480 is formed into a desired shape by being bent to be fitted into the groove of the bending die 490 under the tension. To bend the work piece 480 to be fitted into the groove of the bending die 490, swinging movement (arrow A) of the arms 440, horizontal movement (arrow B) of the pillars to and from the support shafts 430, and vertical movement (arrow C) of the lifts 460 are utilized.
However, such stretch benders must be built of heavy material in order to ensure rigidity of the device. Especially, since each pillar 450 is supported by a single arm 440 in a slidable manner, it is necessary to adopt material having a large cross section. Furthermore, in order to drive the pillars 450 and the lifts 460, high-powered motors are required.
An object of the present invention is to provide a stretch bender which can maintain desired rigidity without using heavy material.
Another object of the present invention is to provide a stretch bender which can perform accurate bending using low-powered drive sources.
Further object of the present invention is to provide a stretch bender which is easy to operate.
To attain these and other objects, the present invention provides a stretch bender for bending a strip work piece by thrusting it against a bending die while applying axial tension to the work piece, comprising a pair of support shafts, and a pair of rotating members rotatably supported by the support shafts and provided with two arm members and a coupling member for connecting the arm members, each of the arm members extending outward from the support shafts.
Each of the rotating members comprises a guide member extending between the two arm members which can move to a direction parallel to the longitudinal direction of the arm members, a chuck mechanism, for gripping the strip work piece, arranged movable to a direction orthogonal to the longitudinal direction of the arm members, and a tension application mechanism for moving the guide member with respect to the arm members and applying tension to the work piece gripped by the chuck mechanism.
The stretch bender of the present invention further comprises rotation control means for controlling rotation of the rotating members in order to thrust the work piece gripped by the chuck mechanisms against the bending die.
High rigidity of the device is realized by the U-shaped rotating members which are constituted of the arm members, parallel to each other and supporting the guide member which holds the chuck mechanism gripping the work piece, and the coupling member by means of which the arm members are connected.
Accordingly, the stretch bender of the present invention can be built without using heavy material and also without deteriorating bending accuracy.
The tension application mechanism may comprise a first drive source for moving one end of the guide member along one of the arm members, a second drive source for moving the other end of the guide member along the other arm member, a tension sensor for detecting tension applied to the work piece, a position sensor for detecting a position of the guide member, and control means for controlling torque of the first drive source based on the tension detected by the tension sensor and also controlling the second drive source based on the position detected by the position sensor.
If the tension application mechanism is constituted as such, drive force for moving the guide member can be shared between the two drive sources having low power. In addition, since the first drive source of the tension application mechanism is controlled based on the tension and the second drive source is controlled based on the position of the guide member, the guide member can be moved with high accuracy by a pair of drive sources while desired tension is applied to the work piece.
The control means may adjust the torque of the first drive source to predetermined target torque based on the tension detected by the tension sensor. Additionally, the control means may comprise storage means for storing the position of the guide member detected by the position sensor and a control value for adjusting the torque of the first drive source to the target torque. It is also possible to provide position control means which, after stretch bending of one work piece, controls the first drive source according to the control value stored in the storage means instead of the control means.
It is preferable that a table on which the bending die is mounted is provided in a tiltable manner. As such, even if the position of the chuck mechanism on the guiding member is reached to its upper or lower limit and further move is restrained, it is possible to place the chuck mechanism at a desired position with respect to the bending die by tilting the table. Thus, it is possible to expand the range of position within which the chuck mechanism can be placed with respect to the bending die and continue bending without interruption. In other words, unless the table is tiltable, the work piece has to be taken off from the chuck mechanism when the position of the chuck mechanism is reached to its upper or lower limit, and the work piece is again attached to the chuck mechanism with its chuck position modified. The above constitution can save such time and labor.
It is preferable that when an operator performs teaching, bending is performed as taught. In this way, bending becomes easy to perform.
The drive sources for driving the guide member may be selectively operated either in a torque control mode or in a position control mode. In this case, the drive sources are driven in the torque control mode to generate predetermined tension, and the positions that the guide member takes at that time are stored in the storage means. In the position control mode, the drive sources are controlled based on the position data stored in the storage means. In initial stretch bending, the guide member is moved in the torque control mode to obtain desired tension. However, once the position of the guide member is fixed, it is possible to perform bending with application of the desired tension only by moving the guide member to the fixed position in the position control mode.